1. Field of the Invention
The present invention relates to a process for improving the quality of lubricating base oils.
2. Description of the Related Art
The "quality" of a lubricating base oil is determined by a combination of properties. Very important properties in this respect are storage stability and filterability of the base oil, and interfacial properties such as demulsibility, air release and foaming tendency. The present invention is particularly concerned with improving the storage stability, demulsibility and filterability of the base oil, whilst other relevant properties like air release and foaming tendency of the base oil are at least not negatively influenced. Under certain conditions air release and/or foaming may even be improved too in the process of the present invention. In any event, the overall quality of the lubricating base oil is improved.
All the aforementioned properties as well as methods for determining their values are well known in the field of base oil manufacture.
The storage stability indicates the number of days for an oil to produce a detectable change, other than a change in color, when stored in the dark at a certain temperature under oxidative conditions, usually in air. This is a very important characteristic of a lubricating base oil, since it gives an indication of how long a lubricating base oil could be stored whilst maintaining free of any deposits, haze or flocculation.
The demulsibility of a lubricating base oil is the ability of this oil to separate from water after the water and the oil have been intimately contacted and agitated so that an emulsion is formed. The demulsibility, accordingly, gives an indication of the rate of coalescence of water drops in the water-oil emulsion. This rate of coalescence, in return, is a good indication of the content of surface-active compounds (i.e. contaminants, hetero-atoms and aromatics) in the base oil, which compounds may originate from their natural occurrence in the fresh oil, from contaminants and/or from degradation reactions taking place during the manufacturing process of the base oil. Demulsibility is determined according to ASTM D1401.
The filterability of a lubricating base oil is a measure of the filter-blocking tendency of this oil. It is an important quality characteristic of a lubricating base oil, since many systems requiring lubrication contain filters whereby plugging of the filters needs to be avoided. Filterability is expressed in terms of the time needed to filter a certain volume of oil through a certain filter under certain conditions. This method for determining the filterability is known as the CETOP filterability method.
The foaming tendency of a lubricating base oil indicates the volume of foam which is generated after bubbling air through the oil for five minutes at a constant rate and temperature and the volume of foam still left ten minutes after the bubbling of air through the oil has stopped. It will be understood that foaming of a lubricating oil during operation may give rise to inadequate lubrication. The standard test method for determining foaming tendency of lubricating oils is ASTM D892.
The air release value of a lubricating base oil indicates the ability of this oil to separate entrained air and is defined as the number of minutes for air entrained in the oil to reduce in volume to 0.2% of its original volume at a certain temperature. A high air release value may indicate that the test oil contains a relatively high amount of air-retaining constituents, such as hetero-atoms (nitrogen, sulphur), polyaromatics and other polar compounds. The air release value is determined according to standard test method IP-313, which is technically identical to ASTM D3427.
In U.S. Pat. No. 4,795,546 a process for improving the storage stability of hydrocracked, catalytically dewaxed lubricating base oils is disclosed comprising a hydrofinishing step followed by a nonhydrogenative stabilization step. The hydrofinishing step involves contacting the dewaxed effluent with hydrogen in the presence of a suitable hydrogenation catalyst under mild hydrogenation conditions. The subsequent nonhydrogenative stabilization step involves contacting the hydrofinished dewaxed oil with a minor amount of an olefinic stabilizing agent in the presence of a heterogeneous acidic catalyst, such as acid resins, clays and aluminosilicates. From the said U.S. specification it becomes clear that the nonhydrogenative acid stabilization must be attributed to a reaction of the olefinic stabilizing agent with the floc forming species rather than to adsorption of these species onto the acidic catalyst. A first drawback of the stabilization method disclosed is the necessity of two distinct process steps, both requiring the presence of a different catalyst. It will be understood that this is undesired from a cost perspective. A further drawback is that the use of a stabilizing agent in the base oil may give rise to blending problems when adding additive packages later on. The olefinic stabilizing agent, namely, could easily interfere with the compounds constituting the additive package, which may give rise to problems with obtaining a stable and uniform blend. The possible interference between olefinic stabilizing agent and additive package may even cause (partial) neutralization of the effect of either the olefinic stabilizing agent or the additive package, which, in return, may have a detrimental effect on the stability of the final lubricating oil.
In European patent application No. 0,535,910 a process for improving the demulsibility of lubricating base oils is disclosed, which process comprises contacting the base oil with an adsorption means, which is either an acidic ion exchange resin or a silica adsorbent. The lubricating base oil is defined as an oil which has been solvent extracted and/or dewaxed and/or hydrotreated. From the disclosure it is, however, clear that the base oil has preferably been solvent extracted prior to contacting with the adsorbent in order to remove aromatic hydrocarbons. It is, however, clear from this patent application that any adsorbent other than an acidic ion exchange resin or silica is not expected to positively affect the base oil's demulsibility performance.
U.S. Pat. No. 4,600,502 relates to a process for decreasing the foaming tendency of lubricating base oils. The process involves passing the base oil through an adsorption zone in order to remove the foam producing compounds, which usually constitute less than 1% by weight of the total weight of the base oil. Before being passed through the adsorption zone the base oil has already been solvent extracted and/or hydrotreated and/or dewaxed in order to remove aromatic compounds. Accordingly, the adsorbent is chosen such, that the adsorption step is solely intended to remove foam producing compounds from the base oil and not to remove any other undesired species, such as certain aromatic compounds. The adsorbents used suitably are neutral or basic, with basic adsorbents being preferred. Among the many basic adsorbents listed, charcoal treated with a solution of a strong base is listed too. However, there is no suggestion that untreated charcoal might be suitable as well.
The processes taught in the prior art usually aim to improve only one single property of a lubricating base oil. The present invention, on the other hand, aims to provide a process for improving the overall quality and in particular the storage stability, demulsibility and filterability of lubricating base oils. Furthermore, the present invention aims to provide a process for improving the quality of lubricating base oils by only one single process step. The present invention also aims to provide a quality-improving process which can be installed and operated at relatively low expenses within existing refinery installations. One aspect in this connection is that the adsorbent to be used should be commercially available at an attractive and competitive price. Yet another aim of the present invention is to provide a process wherein the storage stability of a lubricating base oil is improved without employing any stabilizing agent in view of the addition of any additive packages to the base oil later on, when manufacturing the final tailor-made lubricating oil products.
All these aims have been achieved by the process according to the present invention, which involves improving the overall quality of lubricating base oils via one single adsorption step using dry activated carbon as the adsorbent.
In general, the use of activated carbon as an adsorbent is well known. In the manufacture of hydrocarbon oils, activated carbon is known to be particularly suitable for adsorbing polynuclear aromatic compounds. For instance, U.S. Pat. Nos. 3,697,414; 4,447,315; 4,747,937 and 4,954,242 all describe the use of activated carbon as an adsorbent for removing polynuclear aromatic compounds from different kinds of hydrocarbon streams.
However, none of the aforementioned prior art documents discloses or suggests the use of activated carbon, let alone dry activated carbon, as an adsorbent for improving the overall quality of lubricating base oils in terms of improving storage stability, demulsibility and filterability in one single adsorption step. It has now surprisingly been found that by contacting a lubricating base oil with dry activated carbon, all these properties are improved, whilst at the same time other intrinsic properties of the base oil (air release and foaming tendency) at least reach a commercially acceptable level and under certain conditions are even positively affected.